refactor(#27): IMP-27 K5 catalog loader + _get_block_by_id cleanup

Consolidate three duplicated catalog readers and two _get_block_by_id
implementations behind a single shared module (src/catalog.py) that owns
file-read + mtime cache. All caller signatures and return contracts
remain byte-identical.

Units:
- u1 NEW src/catalog.py (76 lines): load_root_catalog / load_blocks /
  get_block_by_id / get_catalog_mtime as the sole file-read +
  mtime-cache owner.
- u2 src/block_reference.py: _load_catalog delegates to load_blocks
  (list[dict] preserved); _get_block_by_id (no-arg) delegates to
  catalog.get_block_by_id. Module-level _catalog_cache removed.
- u3 src/block_selector.py: load_catalog delegates to load_root_catalog
  (root dict preserved); _get_block_by_id (catalog-injected sig
  preserved) delegates to catalog.get_block_by_id. Module-level
  _catalog_cache / _catalog_mtime / CATALOG_PATH removed.
- u4 src/renderer.py: _load_catalog_map and
  _load_catalog_map_with_variants consume catalog.load_blocks; renderer
  projection caches kept local but keyed via
  catalog.get_catalog_mtime(). Per-projection invalidation keys
  (_CATALOG_MAP_MTIME / _CATALOG_VARIANT_MAP_MTIME) introduced. import
  yaml, CATALOG_PATH, legacy _CATALOG_MTIME removed.
- tests NEW tests/test_catalog_shared_loader.py (421 lines, 23 cases):
  shared loader + 3 wrappers covering single file-read, contract
  preservation, signature preservation, shared cache, private state
  absence, mtime invalidation propagation to renderer projections.

Verification:
- pytest tests/test_catalog_shared_loader.py -v: 23/23 PASS in 0.13s.
- pytest tests/ -q --ignore=tests/matching: 365/365 PASS in 38.10s.
- src/fit_verifier.py, src/space_allocator.py, src/pipeline.py and
  templates/catalog.yaml unchanged (git diff empty).

Out of scope:
- catalog.yaml schema/path unchanged.
- Catalog direct-read call sites in fit_verifier / space_allocator /
  pipeline left for a separate follow-up axis.
- Phase Z 22-step runtime, frame_selection, light_edit/restructure
  flows untouched.

Refs: IMP-27 (gitea #27), INSIGHT-MAP §5 K5, PHASE-Q-AUDIT §2.10

src/block_reference.py

@@ -20,9 +20,10 @@ import re
 from pathlib import Path
 from typing import Any
 
-import yaml
 from jinja2 import Environment, FileSystemLoader
 
+from src import catalog as _catalog_mod
+
 logger = logging.getLogger(__name__)
 
 # 템플릿 디렉토리
@@ -101,32 +102,18 @@ RELATION_CATEGORY_MAP: dict[str, list[str]] = {
 
 
 # ══════════════════════════════════════
-# 카탈로그 로딩 (mtime 캐싱)
+# 카탈로그 로딩 (IMP-27: src.catalog 공유 로더 위임)
 # ══════════════════════════════════════
 
-_catalog_cache: dict[str, Any] = {"data": None, "mtime": 0}
-
 
 def _load_catalog() -> list[dict]:
-    """catalog.yaml 로드 (mtime 캐싱)."""
-    path = TEMPLATES_DIR / "catalog.yaml"
-    mtime = path.stat().st_mtime
-    if _catalog_cache["data"] is not None and _catalog_cache["mtime"] == mtime:
-        return _catalog_cache["data"]
-
-    data = yaml.safe_load(path.read_text(encoding="utf-8"))
-    blocks = data.get("blocks", [])
-    _catalog_cache["data"] = blocks
-    _catalog_cache["mtime"] = mtime
-    return blocks
+    """catalog.yaml blocks list (IMP-27: shared loader delegation)."""
+    return _catalog_mod.load_blocks()
 
 
 def _get_block_by_id(block_id: str) -> dict | None:
-    """블록 ID로 카탈로그 엔트리 조회."""
-    for b in _load_catalog():
-        if b["id"] == block_id:
-            return b
-    return None
+    """블록 ID로 카탈로그 엔트리 조회 (IMP-27: shared loader delegation)."""
+    return _catalog_mod.get_block_by_id(block_id)
 
 
 # ══════════════════════════════════════